The evidence for a critical role for estrogen (and prolactin) in human breast cancer has been slowly accumulating, but remains somewhat controversial. In this proposal we request funds to conduct four studies of relevance to breast cancer etiology. The first study is a comparison of estrogen levels (and their bioavailability) and prolactin in premenopausal breast cancer cases under age 36 and control women. The second study is a comparison of estrogen levels (and their bioavailability) and prolactin in postmenopausal breast cancer cases and controls. The third study is to measure circulating estrogens (and their bioavailability) and prolactin in high risk family members of bilateral breast cancer patients under age 50 and to compare the measured levels to those in control women. The fourth is a study to determine if the protective effect of first full-term pregnancy is mediated in part by decreased bioavailability of estrogens in parous compared to nulliparous women (we have previously shown that prolactin levels are decreased by a first full-term pregnancy). Premenopausal breast cancer cases under age 36 and bilateral breast cancer cases under age 50 have been identified through the Cancer Surveillance Program, a population based tumor registry of Los Angeles County. Family members are identified by having cases complete a mailed questionnaire. Controls for both studies are neighborhood controls selected by a predetermined algorithm. Cases in the second study are identified through a tumor registry which we maintain at a retirement community near Los Angeles. Controls are neighborhood controls closely matched on multiple breast cancer risk factors. Nulliparous and parous women in the fourth study represent a group of Catholic nuns and their biologic sisters. Finally, in this proposal, we request funds to study circulating levels of androgens (and their bioavailability) and estrogens (and their bioavailability) in prostate cancer cases and controls, and to study hormone profiles of young Black and White men in Los Angeles to see if the difference in prostate cancer rates is evident in differences in the hormones responsible for prostatic tissue growth.